The modal windows on our site have a Close button (X) in the upper right corner, and for years that's been the only way to close them. I just added the ability to close modals by pressing the Esc key. I want to let our longterm users (and our new users, too) know that this functionality is available. Is there a good / standard way to show that hint? Maybe as a title on the Close button? Are there any examples of where this is done well?
I would add in the functionality and let users find it organically. You could add a tooltip, since it's a one-time learn, but unless you have many modals that a user has to navigate through on a regular basis, learning to close with esc instead of a click isn't a big win and almost results in a wash.